In a mammal such as a human, an odorous substance is recognized by an olfactory receptor of an olfactory neuron present in olfactory epithelium spread in the deepest portion of a nasal cavity. An odor molecule taken into the nasal cavity works on and activates the olfactory receptor (OR), and a signal from the olfactory sensory neuron caused by the activated olfactory receptor is transmitted to the central nervous system, and thus, an odor is perceived. As for a human, it is presumed that there are 400 or more genes encoding olfactory receptors. It is regarded that the odor quality perceived by a human from a specific odorous substance is determined depending on which combination of 400 or more olfactory receptors is activated.
There are already a variety of reports on the relationship between an odor and olfactory receptors relating to the recognition of the odor. For example, it is suggested, through studies using a mouse made in 2014, that a musk odor which is one of several hundred thousand odorous substances, is recognized exceptionally by a very small number of olfactory receptors. As for a human, merely one olfactory receptor OR5AN1 has been found as the olfactory receptor recognizing musk, and it is suggested that this one receptor makes a large contribution to the recognition of the musk odor (Non Patent Literature 1). Besides, it is suggested that, for example, one olfactory receptor designated as OR10G4 makes a large contribution to recognition of an odorous substance designated as guaiacol (2-methoxyphenol) (Non Patent Literature 2).
In accordance with recent increase in attention to hygiene, various offensive odors, such as a “musty odor” generated in textile products such as clothes, “body odors” generated from the head, the mouth, the groin, the sole and the like, and a “urine odor” generated from urine excreted a long time ago, have become a problem. Many of odorous substances corresponding to causes of these offensive odors are volatile low molecular weight organic compounds having a molecular weight of about 30 to 300. As a method for reducing discomfort against an offensive odor, chemical deodorization utilizing a neutralization reaction, physical deodorization of adsorbing an offensive odorous substance to a porous surface or the like, or sensuous deodorization utilizing a different odorous substance may be employed. Examples of the sensuous deodorization include (1) a method in which another odorous substance is introduced into an environment of an offensive odor to be smelled together with the offensive odor (masking), and (2) a method in which olfactory sensitivity of a human to an offensive odor is lowered by causing another odorous substance to be smelled before smelling the offensive odor (cross-adaptation).
In recent years, efforts have been made to find out an olfactory mechanism of causing sensuous deodorization for efficiently developing a sensuous deodorization technique based on the finding. For example, it has been reported that an odor molecule activating a given olfactory receptor works as an antagonist to inhibit activation of another olfactory receptor. On the basis of this fact, a technique utilizing an antagonist of an olfactory receptor recognizing an offensive odor has been reported as a kind of the sensuous deodorization (1) by masking (Patent Literature 1). Currently, this technique utilizing an antagonist is the only one sensuous deodorization technique from the viewpoint of directly controlling an olfactory receptor.
The method of the sensuous deodorization (2) is a method on the basis of a physiological phenomenon of the olfactory cross-adaptation. The olfactory cross-adaptation is defined as a reduction in sensitivity to an odorant due to habituation to a different odorant following habituation to an odorant. In other words, when one is caused to get used to an odor not offensive, he/she gets used to an offensive odor and hence his/her perception is suppressed. Patent Literature 2 describes an odor-suppressing method based on the cross-adaptation in which the olfactory sensitivity to an underarm odor component is lowered by causing one to continuously smell a substance having an odor which has a chemical structure very similar to that of a causative component of the underarm odor but is weak or not discomfort. Besides, Patent Literature 3 describes a method for evaluating, based on the cross-adaptation, odor similarity between a natural flavor and an imitation flavor imitating the natural flavor. In Patent Literature 3, it is presumed that stronger cross-adaptation is induced as two flavor compositions have odors with more similar odor quality and that the influence of the adaptation is expressed as change in cerebral blood flow, and the similarity of the two flavors is evaluated by using the change in cerebral blood flow as an index.
The lowering of odor sensitivity through the cross-adaptation can be caused not only at the level of olfactory receptors but also through various mechanisms at the level of olfactory neurons, at the level of neural networks and the like. Various hypotheses have been formed as the mechanism of the olfactory cross-adaptation. In one of the hypotheses, the cross-adaptation is regarded to occur in a neuron present in a high-order brain region. Specifically, it is regarded as a mechanism in which the neuron present in the higher-order brain region, where information from peripheral olfactory neurons is integrated to read the nature of an odor, loses its sensitivity so as not to continuously make unnecessary response to odor information remaining without changing. In another hypothesis, the cross-adaptation is regarded to occur at the level of olfactory receptors of peripheral olfactory neurons. Specifically, it is presumed as a mechanism in which an olfactory receptor for a given odorous substance is desensitized as a result of precedently responding to another substance, and hence does not transmit a signal with respect to the odorous substance to which the receptor is exposed afterward. The former hypothesis is difficult to verify because it is still unknown by which high-order brain neural network the odor information is processed. On the other hand, the latter hypothesis is also difficult to verify because of the large number of olfactory receptors, which is said to be 400 or more, and complexity of their combinations in the recognition of odorous substances.
Besides, it is presumed that a substance inducing the cross-adaptation of a given odor is a substance having a very similar chemical structure to a causative substance of the odor or having similar odor quality (Non Patent Literature 3). On the other hand, it is also pointed out that either of these principles does not hold true (Non Patent Literature 4). Non Patent Literature 5 reports that odorous substances accepted by one rat olfactory receptor had similar odor quality, and that the odorous substances induced the cross-adaptation in a human. In general, however, one odorous substance is recognized by a large number of olfactory receptors having different selectivity.
In this manner, the biological mechanism of inducing the cross-adaptation has not been cleared yet, and a method for identifying odorous substances inducing the cross-adaptation has not been established yet. Accordingly, development of a technique to control an odor by artificially and systematically inducing the cross-adaptation has been conventionally very difficult.    (Patent Literature 1) JP-B-5646255    (Patent Literature 2) JP-A-2005-53887    (Patent Literature 3) JP-B-4966790    (Non Patent Literature 1) Shirasu M. et al., Neuron, 81:165-178, 2014    (Non Patent Literature 2) Mainland J. D. et al., Nat. Neurosci., 17(1): 114-20, 2014    (Non Patent Literature 3) Michiaki Kawasaki, Tetsushiro Horiuchi, “Kyukaku to Nioi Busshitsu (Olfactory and Odorous Substances)”, p. 71-72, Japan Association on Odor Environment, 1998    (Non Patent Literature 4) Pierce J. D., Chemical senses, 21:223-237, 1996    (Non Patent Literature 5) Chemosensory Perception, 3(3): 149-155, 2010